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December 9, 2013

Intra Compression Efficiency in VP9 and HEVC

In this paper we get into detailed overview of intra compression data-flow in HEVC and VP9. We describe common and unique stages of both standards. Then we carry out experiments with JCT-VC HM and WebM VP9 encoders on intra compression efficiency.
We also turn some of the HEVC features off to get its dataflow as close to VP9 as possible. Finally we get into discussion of the efficiency of both codecs, the corresponding standards and their intra compression algorithms.

Introduction

The current industrial video compression standard H.264/AVC was adopted in 2003. It provides a superior video compression efficiency compared to other existing and widely spread standards such as MPEG2 or VP8. However the amount of video data stored on local devices or transmitted over the networks is permanently increased. According to Cisco [2] mobile video traffic was 51 percent of the entire global Internet traffic by the end of 2012 and it is expected to be 66 percent by 2017. The ability to get better compression rates will eventually decrease network bandwidth load.

The emerging of a more efficient next generation video coding standard is of a high demand at the moment. There seem to be two main contenders for the position of the next state-of-the-art video compression standard: JCT-VC H.265/HEVC and Google VP9. HEVC is being developed by JCT-VC group - the creators of AVC. It is an evolution of AVC concepts with some innovations. On the other hand, VP9 is a Google initiative to get a royalty-free compression standard with efficiency superior to AVC. It expands techniques used in AVC and VP8 and is very likely to replace AVC at least in the YouTube video service.

The basis of any video compression standard is intra-frame coding that determines the resulting compression efficiency. In this paper we present a detailed overview of intra-frame compression techniques used in VP9 and HEVC. Then we analyze their unique parts and carry out experiments on intra-frame compression efficiency of HM and WebM VP9 encoders.

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Conclusion

Both VP9 and HEVC compression standards provide higher compressoin efficiency compared to the current industrial video compression standard AVC. HEVC provides better compression rates than VP9, but VP9 is patent-free and can be used without licensing expenses.
In our experiments we tried to figure out the main reasons for HEVC to be more efficient in intra coding compared to VP9. We showed that more angular intra prediction modes provide the most significant influence on intra compression efficiency (about 7.7% bitrate savings). The HEVC SAO post-processing stage has less impact (about 1.3% bitrate savings). Finally the modified HM encoder with 10 intra prediction modes and without SAO post-processing is still about 7% more efficient compared to the VP9 encoder. We assume this is due to the adaptive HEVC entropy coding. Syntax-adaptive BAC should be more efficient than frame-adaptive bool-coder, and this topic is a subject for further research.
Further experiments should also be carried out on inter-compression efficiency of both standards.